Igneous

River Cobble (likely Basalt)

Mafic Igneous Rock (Aphanitic)

Hardness: 5-6 (Mohs scale); Color: Dark grey to greenish-grey; Luster: Dull/Earthy (weathered); Crystal structure: Fine-grained/Aphanitic; Cleavage: Generally poor; Specific Gravity: 2.8 - 3.0

Hardness: 5-6 (Mohs scale)
Color: Dark grey to greenish-grey
Luster: Dull/Earthy (weathered)

Identified March 18, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 5-6 (Mohs scale); Color: Dark grey to greenish-grey; Luster: Dull/Earthy (weathered); Crystal structure: Fine-grained/Aphanitic; Cleavage: Generally poor; Specific Gravity: 2.8 - 3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. This specific specimen has been significantly rounded and smoothed by fluvial (river) or glacial transport over thousands of years.

Uses & applications

Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and decorative landscaping stones.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Smoothed river rocks like this one are often referred to as 'cobbles' when they are between 64 and 256 millimeters in diameter.

Field identification & locations

Identify by its heavy weight (density), fine-grained texture, and smooth, water-worn exterior. Found globally in riverbeds, beaches, and glacial till. Collectors often look for unique shapes for rock painting or garden accents.

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral